199 research outputs found
PetrologĂa y geoquĂmica de las tonalitas de Villarejo de MontalbĂĄn
[ES] Las tonalitas de Villarejo de MontalbĂĄn son rocas ricas en biotita en las que se observan segregados mĂĄs fĂ©lsicos con abundante titanita. Los datos geoquĂmicos e isotĂłpicos de roca total ((87Sr/86Sr)300 = 0,7084-0,7089; ΔNd = -4,79, -4,80) sugieren que el magma tonalĂtico se formĂł por fusiĂłn de rocas metaĂgneas de composiciĂłn bĂĄsica/intermedia (anfibolitas o granulitas). A su vez, los segregados fĂ©lsicos parecen haberse formado por acumulaciĂłn del magma residual diferenciado a partir de una matriz formada sobre todo por plagioclasa y biotita. Las condiciones de cristalizaciĂłn estimadas por geotermobarometrĂa del anfĂbol indican temperaturas de 890- 920ÂșC, y presiones de 1,3-2,4 kbar, las cuales implican niveles aproximados de emplazamiento cercanos a los 5-10 km de profundidad. [EN] The tonalites of Villarejo de MontalbĂĄn are biotite-rich rocks showing felsic segregates with abundant titanite. Whole-rock geochemical and isotopic data ((87Sr/86Sr)300 = 0.7084-0.7089; ΔNd = -4.79, -4.80) suggest that the tonalitic magma was formed by melting of metaigneous rocks of basic/intermediate composition (amphibolites or granulites). At the same time, the felsic segregates seem to have formed through the accumulation of a differentiated residual magma after crystal fractionation of plagioclase and biotite. Crystallization conditions estimated according to amphibole geothermobarometry indicate temperatures between 890 and 920ÂșC, and pressures in the range 1.3-2.4 kbar, which is in accordance with an approximate emplacement level close to 5-10 km deep.Peer Reviewe
Eclogite facies relics in metabasites from the Sierra de Guadarrama (Spanish Central System): P-T estimations and implications for the Hercynian evolution
Relics of HP-MT eclogitic assemblages related to the first metamorphic stage of the Hercynian orogeny
in the Sierra de Guadarrama (Spanish Central System, SCS) are preserved as boudins of pre-Ordovician
metabasites enclosed by felsic gneisses. Textures indicate a multi-stage metamorphic history starting in
the MT eclogite facies (as deduced from the presence of omphacite and rutile included in garnet) and
continuing through medium to low pressure granulite and retrograde amphibolite-greenschist facies.
Thermobarometric calculations in the eclogitic paragenesis yield pressures of ~14 kbar for temperatures
in the range 725 7758C. Thermobarometry for the subsequent granulitic stage indicates a significant
drop in pressure (P < 10 kbar) for similar temperatures of ~7508C. Metabasites vary from gabbro to
leucotonalites showing the typical Fe enrichment of the tholeiitic series. Chemical characteristics
indicate a derivation from low-pressure crystallization of tholeiitic melts more enriched than typical
MORB compositions. Their original location far from continental margins as evidenced by the absence
of ophiolitic material in the area and their association with platform sediments suggests that
eclogitization was related to intracontinental crustal subduction and thickening. The P-T conditions
estimated in the metabasites for the first metamorphic stage are similar to ones deduced for the
surrounding metasediments and suggest that the Hercynian crust could have reached a thickness of
~70 80 km, which is more than the double the present thickness
Origin, ore forming fluid evolution and timing of the LogrosĂĄn Sn-(W) ore deposits (Central Iberian Zone, Spain)
The LogrosĂĄn Snâ(W) ore deposits in the metallogenic SnâW province of the European Variscan Belt consist of endo- and exogranitic greisen-type and quartzâcassiterite veins associated with a S-type granite. Mineral characterization, fluid inclusion study, isotope geochemistry and ArâAr geochronology have been combined in order to reconstruct the conditions for Snâ(W) mineralization. The endo- and exogranitic mineralization must have been developed in a relatively long-lived system (~Â 308â303Â Ma), during or soon after the emplacement of the LogrosĂĄn related-granite (at ca. 308Â Ma). The mineralizing fluids are characterized by complex aqueous and volatile (H2OâN2âCO2âCH4âNaCl) fluid inclusions. Microthermometry and Raman analyses indicate that fluid composition evolved from N2âCH4 to N2-rich, followed by CO2-rich fluids, with varying amounts of H2O. The presence of N2 and CH4 suggests the interaction with fluids derived from the nearby metasedimentary host rocks. A model of host-rock interaction, assimilation, and mixing of metamorphic and magmatic fluids, resulting in change of the redox conditions, is proposed for tin deposition. Later sulfide minerals were precipitated as a result of pressure and temperature release
Relaciones entre ortogneises y series volcano-sedimentarias en el macizo de El Caloco (Guadarrama Central)
El macizo de El Caloco estĂĄ constituido por dos formaciones metamĂłrficas
de caracteres distintos. Una es fundamentalmente paradcrivada
(metapelitas con niveles carbonatados asociados>, con tramos
de origen voleano-sedimentarios o subvolcĂĄnico (gneises glandulares
fémicos) y que se ha denominado «serie fémica heterogénea».
Aparecen también metabasitas con clinopiroxeno, granate y plagioclasa,
en parte anfibolitizadas.
La otra formaciĂłn es de naturaleza cuarzo-feldespĂĄtica y estĂĄ compuesta
por ortogneises glandulares y leucogneises de origen mctagranĂtico.
En funciĂłn de los caracteres petrolĂłgicos y geoquimicos
interpretamos los ortogneises cuarzo-feldespĂĄticos como representantes
de antiguos granitos de dos micas con silicatos de aluminio (granitos
tipo 8). El estudio geoquimico pone de manifiesto en las facies
mĂĄs diferenciadas (leucogneises con nidos de turmalina y/o
granate> un trend evolutivo de carĂĄcter sĂlico-potĂĄsico.
Existe un contacto neto y discordante entre ambos conjuntos, encontrĂĄndose
localmente facies mosqueadas en los esquistos metapelĂticos
prĂłximos al mismo.Peer reviewe
EvoluciĂłn metamĂłrfica del sector centro-septentrional de la Sierra de Guadarrama
Tesis Univ. Complutense de Madrid.Depto. de MineralogĂa y PetrologĂaFac. de Ciencias GeolĂłgicasTRUEProQuestpu
Mineralogical and isotopic characterization of graphite deposits from the Anatectic Complex of Toledo, central Spain
Graphite is found dispersed in high-grade metapelitic rocks of the Anatectic Complex of Toledo (ACT) and was mined during the mid twentieth century in places where it has been concentrated (Guadamur and la Puebla de MontalbĂĄn mines). Some samples from these mines show variable but significant alteration intensity, reaching very low-T hydrothermal (supergene) conditions for some samples from the waste heap of the Guadamur site (<100 °C and 1 kbar). Micro-Raman and XRD data indicate that all the studied ACT graphite is of high crystallinity irrespective of the degree of hydrothermal alteration. Chemical differences were obtained for graphite ÎŽ13C composition. ACT granulitic graphite shows ÎŽ13CPDB values in the range of â20.5 to â27.8ââ°, indicating a biogenic origin. Interaction of graphite with hydrothermal fluids does not modify isotopic compositions even in the most transformed samples from mining sites. The different isotopic signatures of graphite from the mining sites reflect its contrasted primary carbon source. The high crystallinity of studied graphite makes this area of central Spain suitable for graphitic exploration and its potential exploitation, due to the low carbon content required for its viability and its strategic applications in advanced technologies, such as graphene synthesis.This work is included in the objectives of, and supported by, the CGL2012-32822 project of the Ministerio de EconomĂa y Competitividad of Spain and the 910492-UCM group.Peer reviewe
Geochemistry and geochronology of mafic rocks from the Spanish Central System: Constraints on the mantle evolution beneath central Spain
The Spanish Central System (SCS) contains several suites of Palaeozoic mafic igneous intrusions with contrasting geochemical affinity: Ordovician tholeiitic metabasites, Variscan calc-alkaline gabbros (Gb1) and microdiorites (Gb2), shoshonitic monzogabbros (Gb3) and alkaline diabases and lamprophyres (Gb4). Not all of these rocks are accurately dated, and several aspects of their genesis are still poorly understood. We present new whole-rock geochemical data (major and trace elements, and SrâNd isotopes), UâPb and LuâHf isotopic ratios on magmatic zircons and 40Ar/39Ar amphibole geochronology results in order to establish a precise chronology for the successive events of magmatism in the SCS, and discuss the nature of their mantle sources. Accurate ages have been determined for the Variscan gabbros (305â294 âMa), the microdiorites (299 âMa) and the accompanying felsic porphyries (292 âMa), the shoshonitic monzogabbros (285 âMa), and the alkaline diabases (274 âMa) and monzosyenites (271â264 âMa). According to this information, the Variscan mafic magmatism would be mainly concentrated in the range of 305â294 âMa, with a final manifestation represented by the minor shoshonitic dykes. The alkaline magmatism proved to be slightly older than previously thought and yielded at least two distinct pulses: diabases and lamprophyresâmonzosyenites. Zircon Hf isotopes evidence the involvement of depleted and slightly enriched mantle sources. The bulk of the ΔHf values are in the broad range of â8 to +11, indicative of melting both depleted and enriched mantle regions. The high within-sample Hf isotope variation (up to ~11 epsilon units) shown by samples from the Variscan series (gabbros, microdiorites and monzogabbros) could be explained mainly by hybridisation of magmas derived from heterogeneous lithospheric mantle sources. Pressure estimates indicate that the Variscan mafic magmas were extracted from the lithosphere. The NdâHf isotopic composition of these suites of rocks suggests the recycling of pelitic sediments during the Cadomian orogeny. Deeper (asthenospheric) mantle levels were involved in the generation of the alkaline suite, whose anomalous negative ΔHf values (moderately decoupled with respect to radiogenic Nd) could be associated with subducted oceanic components raised by mantle upwelling associated with lithosphere thinning and extension during the Permian
Xenolitos peridotĂticos del volcĂĄn MorrĂłn de Villamayor (Campo VolcĂĄnico de Calatrava)
The El MorrĂłn de Villamayor (MVM) peridote xenoliths vary
from orthopyroxene-poor lherzolite to wehrlite in modal composition. This compositional feature contrasts with other Calatrava (CVF)
xenolith suites. The studied xenoliths equilibrated at lower temperatures (618-942 o
C) and slightly shallower (8.8-13.6 kbar) conditions
than other CVF peridotites. MVM peridotites show local intense interaction with the host leucitite displaying spongy rims around primary clinopyroxene and also reaction zones with K-rich minerals
(e.g., sanidine, leucite and richterite) and widespread clinopyroxene,
olivine and spinel neoblasts. Nevertheless, the orthopyroxene-poor
character of MVM peridotites might be caused by some previous
metasomatic eventLos xenolitos peridotĂticos del volcĂĄn El MorrĂłn de Villamayor
(MVM) son lherzolitas pobres en ortopiroxeno y wehrlitas, en contraste con los datos previos de xenolitos de otros volcanes de Calatrava. Las peridotitas estudiadas estĂĄn equilibradas a temperaturas
mĂĄs bajas (618-942 o
C) y menor profundidad (8,8-13,6 kbar) que las
estimadas en otras peridotitas del Campo VolcĂĄnico de Calatrava.
Localmente hay una intensa interacciĂłn del fundido leucitĂtico con
los minerales primarios de la peridotita, ya que se originan zonas
de reacciĂłn con minerales ricos en K (p.ej., sanidina, leucita y richterita) y frecuentes neoblastos de clinopiroxeno, olivino y espinela. El
carĂĄcter pobre en ortopiroxeno de las peridotitas MVM debe estar
causado por algĂșn evento metasomĂĄtico previ
Geothermobarometric applications of trace-element contents in granulite minerals: Zr-in rutile and HREE-in garnet from lower crustal felsic xenoliths of the Spanish Central System
Geothermobarometric estimations based on the trace element concentration of minerals from lower crustal
peraluminous felsic granulite xenoliths from the Spanish Central System (SCS) have been performed. Zr in
rutile thermometry (Zack et al., 2004) gives a main range of 970 to 1100 ÂșC whereas HREE in garnet
barometry (Bea et al., 1997) yields a pressure range of 7.2 to 9.3 kbar. These new thermobarometric data
overlap previous P-T estimates based on conventional metamorphic phase equilibria. Nevertheless, the
slightly higher temperature range obtained with rutile thermometry force to future revision of granulite
mineral equilibria of the SCS lower crustal xenolith
Pyroxenites and Megacrysts From Alkaline Melts of the Calatrava Volcanic Field (Central Spain): Inferences From Trace Element Geochemistry and Sr-Nd Isotope Composition
Alkaline volcanic rocks from explosive monogenetic centers often carry an unusual cargo of crystals and rock fragments, which may provide valuable constraints on magma source, ascent and eruption. One of such examples is the Cenozoic Calatrava Volcanic Field in central Spain, a still poorly explored area to address these issues. Clinopyroxene, amphibole and phlogopite appear either as megacryst/phenocrysts or forming fine-grained cumulates (pyroxenite enclaves s.l.) in some eruptive centers of this volcanic field. They have previously been interpreted as cogenetic high-P minerals formed within the upper lithospheric mantle. The presence of Fe-Na-rich green and Mg-Cr-rich colorless clinopyroxene types as phenocryst cores or as oscillatory zoned crystals in pyroxenite enclaves points to a complex evolution of mineral fractionates from petrogenetically related magmas. In trace element chemistry all studied clinopyroxene types show parallel rare earth element patterns irrespective of whether they are megacrysts, colorless or green core phenocrysts, or zoned crystals within pyroxenite cumulates. This similarity indicates a genetic relationship between all the fractionated minerals. This is in agreement with the overlapping of initial 143Nd/144Nd and 87Sr/86Sr ratios of pyroxenite enclaves (0.512793â0.512885 and 0.703268â0.703778) that is within the chemical field of the host magmas and the Calatrava volcanics. The initial 143Nd/144Nd and 87Sr/86Sr ratios of megacrystic clinopyroxene, amphibole and phlogopite show a more restricted range (0.512832â0.512890 and 0.703217â0.703466), also falling within the isotopic composition of the Calatrava volcanic rocks. Deep magmatic systems beneath monogenetic volcanic fields involve several stages of melt accumulation, fractionation and contamination at variable depths. Trace element and isotope mineral chemistry are powerful tools to understand the history of ascent and stagnation of alkaline basaltic magmas and discriminate between magma mixing, wall-rock contamination and closed magmatic system evolution. In our study, we establish a cogenetic origin for green and colorless clinopyroxene as high-pressure precipitates from liquids of different fractionation degrees (up to 80%, for the highly evolved melts equilibrated with the green clinopyroxene), originated from a highly solidified front of silica-undersaturated alkaline magmas at mantle reservoirs
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